<!DOCTYPE html>
<style>
  #slidein {
    width: 200px;
    height: 200px;
    transform: translate(200px, 0);
  }

  #header {
    height: 50px;
    background-color: green;
  }

  #section {
    width: 200px;
    height: 200px;
    background-color: blue;
  }
</style>

<div id="slidein">
  <div id="section">
    <div id="header">This div should stay in front.</div>
  </div>
</div>
